This invention generally relates to switching citcuits for differential amplifiers, and more particularly a switching circuit for a differential amplifier suitable for being manufactured through integrated circuit technique and having a small variation in the output offset and a small current variation.
One example of a prior art switching circuit of a differential amplifier is shown in FIG. 1. As shown, there are provided a pair of NPN transistors Q1 and Q2 with their emitter electrodes commonly connected, and an NPN transistor Q3 with its emitter electrode connected to ground through a resistor R1 for constituting a constant current source. Resistors R2 and R3 constitute load resistors of a differential amplifier constituted by transistors Q1 and Q2. The load resistors are connected, at one end, to the collector electrodes of the transistors Q1 and Q2, while they are commonly connected, at the other end, to a source V.sub.cc. Characters IN1 and IN2 designate input terminals to which input signals are applied, OUT1 and OUT2 designate output terminals and CT a terminal to which a switching signal is applied.
With the switching circuit for the differential amplifier described above, when a switching signal is supplied to the base electrode of the transistor Q3 that constitute the constant current source, transistors Q1 and Q2 that comprise the differential amplifier are respectively switched to produce a switched output signal from their collector electrodes corresponding to the switching signal.
With this switching circuit, however, when the transistors Q1 and Q2 are cut off by the switching signal, their collector voltage rises to the voltage of the source V.sub.cc which not only affects succeeding circuits but also produces transient noise. Moreover, variation in the current caused by the switching is large which gives a switching noise to a circuit connected to the output OUT1 or OUT2 through a line impedance. Furthermore, when the transistors Q1 and Q2 are cut off, the collector impedance of the differential amplifier increases thus causing leakage from the base electrodes.